The basic construction of random orbital sanders is well known and typically comprises an essentially circular sanding disc and pad having a central mounting through a freely rotatable bearing eccentrically mounted on the end of a drive spindle. Rotation of the drive spindle causes the sanding disc to orbit about the drive spindle. When no external forces act on the disc, the inherent friction in the bearing results in the disc tending to rotate about the spindle axis at full spindle rotation speed. On the other hand, when light pressure is applied to the sanding disc, rotation of the disc can be prevented and the disc merely orbits as, for example, in a conventional orbit pad sanding machine.
When the sanding pad is engaged with a workpiece surface, the frictional contact between the pad and the workpiece results in a pad rotation at a speed considerably less than the speed of spindle rotation. The pad will rotate in a direction opposite the direction of spindle rotation. This rotation combined with pad orbital movement is very useful in achieving a smooth sanded surface.
However, a problem with prior random orbit sanders is that when the sander is operated with no external forces acting on the sanding pad, it rotates at full spindle speed. Thus, the operator has to be extremely careful when applying the pad to a workpiece, otherwise the inertia of the pad will result in a deep gouge being cut in the workpiece before the pad slows to its far less aggressive random orbit movement.
Various attempts have been made to overcome this problem. For example, U.S. Pat. No. 5,317,838, issued to Bourner, discloses a sanding apparatus having a resiliently biased brake mounted in the housing and is adapted to bear against a low friction annular surface of a platen in a direction substantially parallel to the axis of the drive spindle. The brake is a finger brake and includes a body mounted in the housing, a finger slidable in the body, and a spring disposed between said body and a stem of said finger. This brake, however, adds complexity to the sanding apparatus as well as reliability concerns for the extra components and is subject to wear.
Another known sander having a braking member is disclosed in U.S. Pat. No. 5,392,568, issued to Howard, Jr. et al. The braking member includes a base portion, an outwardly flaring, relatively thin wall portion and an enlarged outermost edge portion adapted to frictionally engage an upper surface of the platen. The braking member is secured to the bottom of the shroud of the housing via a groove formed in its base portion. The braking member exerts a relatively constant spring force against the upper surface of the platen which limits the rotational speed of the platen to approximately 1200 rpm when the platen is lifted off of a work surface without significantly degrading the performance of the sander under load. As described above, this braking member also adds complexity to the sander and is subject to wear.